1. Field of the Invention
The present invention relates to a chemical concentration control device for a semiconductor processing apparatus for keeping the concentrations of processing chemicals constant and maintaining the liquid level of chemicals required for processing semiconductors at a certain level.
2. Description of the Related Art
In the semiconductor processing apparatus such as a wet station, chemical concentrations may vary and the chemical quantities may be reduced due to repeated unloading of wafers from a processing bath or due to evaporation of chemicals. Therefore, it is necessary to maintain the concentrations and quantities of chemicals at constant values by adding them periodically.
FIG. 1 is a block diagram showing an apparatus for controlling chemical concentrations for a semiconductor processing apparatus in the related art. In order to maintain chemical concentrations or liquid level at constant values, chemical concentrations are measured with a concentration meter 7 as shown in FIG. 1. A replenishment control unit 40 checks whether or not concentration data measured by the concentration meter 7 falls within a reference range, selects the set values for replenishment for the case where the concentration data falls within the reference range and for the case where it is outside the reference range respectively, issues signals to a replenishment pump (not shown) at a replenishing station 28, and replenishes chemicals as needed. The chemical concentration control device for a semiconductor processing apparatus shown in FIG. 1 constitutes a concentration feedback control system that compares concentration data measured by the concentration meter 7 with the reference range of concentrations and controls chemical concentrations so that they fall within the reference range.
In control of chemical replenishment, as shown in FIG. 2, predetermined quantities of chemicals are replenished based on a replenishment cycle timer built in the replenishment control unit 40 for measuring a certain cycle time and a chemical replenishment timer actuated synchronously with the replenishment cycle timer. FIG. 2 is a timing chart for replenishment control of three types of chemicals (Chemical A, Chemical B, and Chemical C).
As shown in FIG. 2, a replenishment timer for Chemical A, a replenishment timer for Chemical B, and a replenishment timer for Chemical C are actuated based on signals from the replenishment cycle timer in the replenishment control unit 40. The setting time of the replenishment timer for Chemical A is represented by T1, the setting time of the replenishment timer for Chemical B is represented by T2, and the setting time of the replenishment timer for Chemical C is represented by T3. These replenishment timers activate the replenishing pumps provided for the respective chemicals for the periods of time T1, T2 and T3 to replenish certain quantities of chemicals. Chemicals are replenished depending on their concentrations. For example, when the concentration of Chemical A is higher than the reference range and the concentration of Chemical B is lower than the reference range, Chemical A is replenished by a small quantity, and Chemical B is replenished by a large quantity. In this manner, the semiconductor processing apparatus in the related art is adapted to replenish predetermined certain quantities of chemicals to maintain the chemicals at constant concentrations.
However, in chemical replenishment control based on the comparison between concentration data obtained by the concentration meter and the reference range of concentration as a target value conducted by the replenishment control unit 40 in the related art, since concentration data (P) is settled at the timing t=m2 after chemicals are replenished at the timing t=0 under the influence of the time required for mixing chemicals or of the time required for measuring chemical concentrations by the concentration meter as shown in FIG. 3, an overhead time m2 (shown in FIG. 4) is generated in the concentration feedback control system.
Therefore, as shown in FIG. 4, when trying to control chemical concentrations so as to fall within the reference range, chemical concentrations may vary because there may be the cases where the chemical concentrations exceed the upper limit of the reference range or underrun the lower limit of the reference range of concentration under the influence of the overhead time m2. Likewise, there may be the case where the concentration cannot be controlled with a high degree of accuracy due to the overhead time m2 even when PID (proportional-plus-integral-plus-derivative) control is employed. Due to variations in chemical concentrations, processing such as washing and etching are not stable, which may result in lowering of yielding percentage of the semiconductor. Therefore, it is required to maintain the chemicals in the semiconductor processing apparatus at constant concentrations.